1. Field of the Invention
The present invention relates to a relay apparatus for relaying, in a limited bandwidth, communication by a plurality of wireless terminals in a communication area, for example, as in access points in a wireless local area network (LAN).
2. Description of Related Art
Recently, for example, as in an IP telephone via a wireless LAN, a wireless terminal that performs a telephone conversation via radio communication has become widespread. The wireless terminal that performs a telephone conversation via radio communication enables a telephone conversation with a terminal on a partner side by performing radio communication with a relay apparatus (e.g., an access point of a wireless LAN, etc.). In such a communication form, a plurality of wireless terminals are present in a communication area of a relay apparatus, whereby simultaneous telephone conversations by the wireless terminals occur. In this case, the plurality of wireless terminals share a bandwidth available in the relay apparatus, and perform communication respectively.
Furthermore, generally, in radio communication between a wireless terminal and a relay apparatus, a packet transmission error rate is high due to the influence by an obstacle and the like. In the case where the relay apparatus fails in the transmission of a packet, it retransmits the packet until it succeeds in transmitting the packet. When a packet is retransmitted, a bandwidth in use increases accordingly, and when a bandwidth in use exceeds the capacity of transmission which relay apparatus manages, transmissions of radio terminals and relay apparatus interfere each other, and tend to end in failures. Therefore, the number of retransmissions should be as small as possible.
Conventionally, a method for preventing the number of retransmissions from increasing excessively in an OFDM-CDMA transmitter is proposed (e.g., see JP 2004-187226 A). According to this method, when the number of retransmissions increases, the increase in the number of retransmissions is prevented by increasing the number of sub-carriers of an OFDM.
However, according to the above-mentioned conventional method, only the communication state among individual apparatuses are taken into consideration. On the other hand, there is a demand for controlling the number of packet retransmissions in accordance with the entire use situation of a bandwidth by radio communication in a communication area. For example, during a peak time such as the simultaneous telephone conversations by a number of wireless terminals in a communication area, unless the number of retransmissions in radio communication by one wireless terminal is suppressed in accordance with an available bandwidth, the available bandwidth may be exhausted due to retransmissions. Consequently, the transmission of communication data of another wireless terminal may be delayed. Furthermore, during an off-peak time in which only one wireless terminal performs communication in a communication area, in spite of the fact that there is a sufficient available bandwidth for retransmissions, retransmissions of a previously set upper limit value or more cannot be performed, resulting in a packet transmission failure. According to the above-mentioned conventional method, since the entire use situation of a bandwidth for radio communication by wireless terminals in a communication area is not taken into consideration, the above-mentioned circumstances cannot be prevented.